The present invention relates to a visually pleasing, soft, and absorbent paper product having improved bulk, absorbency and embossing pattern definition and a method for the manufacture of such a paper product. The present invention also relates to a method of efficiently delivering a treating agent to a web.
In the area of consumer paper products, for example, bathroom tissue, paper towels and napkins, softness, absorbency, and strength are key attributes considered by consumers. It is highly desirable that the paper product have a consumer perceived feel of softness. This softness plays a key role in consumer preference. Softness relates both to the product bulk and surface characteristics. In addition to softness, the consumer desires a product that is both strong and absorbent to minimize the amount of the product which must be used to do an effective job.
Visual impression is known to dominate the other human senses. A consumer faced with a visually pleasing product establishes an expectation for that product, and unless that expectation is baseless, the product rarely fails to live up to the consumers expectation. In other words, a consumer who visually perceives a product to be soft and absorbent almost always finds the product to actually have those characteristics. Embossing designs can impart both nonvisual qualities in terms of bulk and absorbency, as well as visual qualities based upon perception because vision plays such a dominant role in consumer perception.
To improve softness, standard paper making processes often add chemicals, e.g., softeners and debonders, to a fiber furnish or web to improve or change the properties of the web. Traditionally, softeners and debonders are used in the papermaking process to enhance softness or to adjust strength. Typically, these chemicals are added to the wet end of the paper making process, i.e., in the paper making slurry. When used in this manner, these chemicals react with fines, pitch, sand and other materials associated with pulp fibers to form deposits. Deposits negatively impact productivity because they bind fabrics, plug felts and significant expenses must be incurred to remove the deposits. In some instances, the presence of these chemicals requires adjustment of the system pH. Because of the fatty acid groups, hydrophobicity is imparted on the paper product and this renders it non-absorbent. In some cases, additional expense must be incurred when hydrophilic surfactants are used to restore or impart absorbency.
In most cases, the tensile strength is significantly reduced. Either additional energy is used in refining or additional expense is incurred when a dry strength additive is needed for strength adjustment. Either way, the softness gained in this way is compromised because of the inverse relationship between softness and strength. When used in this manner, softeners and debonders, while enhancing softness and bulk, will have no effect on emboss pattern definition.
Alternatively, these chemicals have occasionally been sprayed onto the wet web prior to drying. These processes suffer from the disadvantages of contamination and materials loss since the chemicals are often lost with the moisture removed from the web during the drying process. Chemicals applied in this manner are usually recirculated back to the wet end where they also react with fines, pitch, sand, and other materials associated with the pulp fibers to form deposits and may require pH adjustment. Deposits negatively impact performance and are expensive to remove and clean up. In most cases, tensile strength is significantly reduced.
The interplay of softness and strength have been the focus of much research. U.S. Pat. No. 4,759,530 teaches the creation of soft surface zone/strong zone composites whereby debonder penetration is limited to 40% of the sheet with the use of vacuum suction installed in front of the applicator to control debonder penetration.
Embossing is the act of mechanically working a substrate to cause the substrate to conform under pressure to the depths and contours of a patterned embossing roll. During an embossing process, the roll pattern is imprinted to the web at a certain pressure or penetration depth. Embossing usually results in a paper web having increased caliper or bulk and absorbency; however, this increase is usually accompanied by an increase in the surface roughness or friction deviation and strength decrease of the embossed tissue or towel product. For a given pattern, the amount of caliper generated and how well the pattern is defined on the substrate depends on the pressure applied on the emboss rolls. Embossing reduces the strength of the tissue as the emboss pressure applied to the patterned rolls is increased. By enhancing pattern definition at a fixed penetration depth, the present invention overcomes the aforementioned deficiencies.
In the production of paper products it is known to emboss sheets comprising multiple plies of creped tissue to increase the surface area of the sheets thereby enhancing their bulk and moisture holding capacity. Highly defined emboss patterns are desirable for their aesthetic appearance.
Chemicals have not traditionally been added to a web after drying because the drying process is designed to impart certain characteristics of, for example, stretch and crepe to a cellulose web. When a dried web is rewet, the additional water/moisture increases hydrogen bonding in the web resulting in a web having increased tensile strength; however, the stiffness or rigidity of the web is also increased. In creped structures, the web loses a majority of its stretch, its crepe and also becomes less soft and coarse. Typically, operational problems are also encountered when the web is rewet as it becomes difficult to subject the web to any tension needed to make rolls or to form the web into reels. In addition to the disadvantages outlined above, a rewet web will have to be subjected to an additional drying process.
As can be seen from U.S. Pat. Nos. 2,803,188; 4,073,230 and 4,135,024, the use of water to rewet the sheet and enhance the definition of the embossing pattern is known. Each of these systems use high temperature to set the pattern because of the need to dry the sheet. Since none of these systems controls the droplet size, it is evident that each system causes sheet rewet requiring subsequent drying. As discussed above, this rewet causes significant losses in web characteristics, for example, stretch and crepe, as well as resulting in a sheet that is stiff, coarse and less soft. As used herein stretch is related to crepe. Pulp and Paper: Chemistry and Chemical Technology, 3rd Edition, Vol. 3, Edited by J. P. Casey defines stretch and/or elongation as the amount of distortion that paper undergoes under tensile stress and it is usually measured on the tensile tester at the same time tensile strength is measured.
Emboss definition refers to the contrast between adjacent surfaces created as a result of shadowing. Shadowing is created by relative elevations between the surfaces of a paper web and the abruptness of the change in elevation or topography between the surfaces. Generally, as a web is passed through an emboss nip, some areas of the web in the pattern experience higher levels of densification. Increased densification and opacity created at the top of a protuberance tends to improve the definition of the embossing pattern by enabling the structure to hold its shape. The relative reflectivity and opacity of the surfaces of the web also contribute to the intensity of the shadowing effect which results in improved emboss definition.
While the use of embossing and the use of softening/debonding agents have individually been known for some time, these processes have never been combined as described herein to simultaneously enhance pattern definition, bulk and absorbency in a paper product.
In addition, the present invention overcomes disadvantages in the prior art associated with building strength, bulk, absorbency and softness into a web. Usually, bulk and absorbency can be added to a web but at the expense of softness, particularly surface roughness as measured by friction deviation. With the method according to the present invention, all three, bulk, absorbency and softness in addition to pattern definition can be improved simultaneously without loss or relaxation of stretch and crepe.
The present invention overcomes these and other disadvantages associated with the prior art. The present invention provides both a method for applying a chemical treatment to a dried web and a method for improving the definition of an emboss pattern without the disadvantages of the prior art liquid applications including the need for an added drying step.
The present invention provides a method whereby treating agents may be added to a dried web with the advantages of high solids delivery, precision in material delivery, improvement in web qualities, and high productivity. The present invention provides a method whereby a treating agent can be added to a dried web without web rewet or loss of crepe, stretch or process runnability.
The present invention also provides an improved method of setting an emboss pattern with softening and debonding treatment agents and/or water while maintaining stretch and crepe and improving pattern definition, bulk, and absorbency of the embossed product. Specifically, the present invention provides a method of delivering a treating agent to a cellulose web, preferably having an average pore size distribution of from about 100 to about 1000 xcexcm and a preferred solids content of from about 70% to about 100%, in an average droplet size of no greater than 200 xcexcm. No heat treatment or additional drying of the web is necessary; no adjustment of the pH is necessary; and no adjustment of the penetration depth of emboss roll pressure is necessary.
The present invention also provides a chemi-mechanically softened, absorbent embossed paper product having enhanced softness, pattern definition, bulk and absorbency. The present invention also provides softening and debonding compositions and an emboss process as described to set emboss patterns so that the products with enhanced visual or pattern definition, softness, bulk and absorbency are obtained. All of these attributes being achieved without loss in crepe, stretch, process runnability or the need to increase the penetration depth or pressure in the emboss process.
To achieve the foregoing advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is disclosed:
A method of efficiently delivering a treating agent to a cellulose web including providing a cellulose web having a solids content of at least about 70% and treating the web with a treating agent having an average droplet size not greater than 200 xcexcm.
There is further disclosed:
A method of enhancing emboss definition in a cellulose web without loss of softness including applying to the cellulose web a liquid agent having an average droplet size not greater than 200 xcexcm; embossing the cellulose web; and again applying a liquid agent which may be the same or different and also having an average droplet size not greater than 200 xcexcm.
Further advantages of the invention will be set forth in part in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate various aspects of the invention and, together with the description, serve to explain the principles of the invention.